Blood flow keeps tissue alive. Measuring that flow is an important medical need. Doppler ultrasound methods utilize the change in frequency of ultrasound backscattered from moving blood to deduce the velocity of the scatterers; such Doppler methods are well-known, and used extensively clinically. (See a text entitled, Doppler Ultrasound, by David H Evans and W. Norman McDicken, John Wiley & Sons, Chichester, 2000.)
U.S. published patent application Ser. No. 10/953,276 filed on Sep. 29, 2004 (referred to here as the '276 application, and incorporated herein by reference in its entirety), assigned to the assignee herein, taught means and methods of measuring blood flow within vessels with ultrasound transducers mounted in or upon the vessel wall. These means and methods used the Doppler effect and certain ultrasound transducers, known as diffraction-grating transducers (“DGT”), alone or in association with conventional, non-DGT transducers.
It would be desirable, however, to be able to measure flow in blood vessels without the need to embed the transducers in an artificial graft or place them on the outer wall of a vessel—without the need for surgical incision. Over the last several decades, catheter procedures have been developed to permanently place metallic structures (such as stents to prop open arteries), or temporarily place such structures (such as vena cava filters to stop post-operative blood clots from going to the lung), and now are part of usual medical care, known as minimally invasive procedures.
There are many clinical situations where a physician desires to place a flow sensor by these minimally invasive means, whether permanently to measure blood flow, e.g. in the coronary arteries of the heart, or temporarily, e.g. post-operatively to evaluate the success of a transplant or other major surgical event. Stents and vena cava filters must be small in diameter so they can enter the blood vessel within a catheter, and be flexible to expand to fit against the blood vessel's walls. Rigid, rectangular piezoceramics transducers, as described in the '276 application, cannot easily be incorporated into the required thin, flexible, expandable mesh-like structures needed for minimally invasive stents and vena cava filters.